1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave element and a surface acoustic wave apparatus. More particularly, the present invention relates to a method of manufacturing and adjusting the frequency of a surface acoustic wave element.
2. Description of the Related Art
A conventional frequency adjusting or trimming method for a surface acoustic wave device which reduces the thickness of an interdigital transducer (IDT) or piezoelectric substrate is disclosed in Japanese Unexamined Patent Publication No. 2-189011.
In the conventional method, as shown in FIG. 6A, IDT 21a is selectively etched so as to reduce the thickness of the IDT 21a, thereby increasing the frequency thereof. Alternatively, as shown in FIG. 6B, the upper surface of an exposed piezoelectric substrate 20a is selectively etched so as to partially reduce the thickness of the piezoelectric substrate 20a, thus reducing the frequency.
However, when IDT 21a is made of Al, which is the most common material used for surface acoustic wave devices, reduction in the thickness of the IDT causes very little increase in the frequency. Thus, adjusting the frequency by reducing the thickness of IDTs is not a practical and commercially useful method.
In general, as a method of performing an etching treatment, a wet etching method has been used. However, since the fine size treatment achievable through the use of this method is only up to 1 xcexcm, there is the problem that it is impossible for such a method to be used for treating an IDT having a smaller line width. Moreover, since the precision achieved with an etching method is relatively low, it is likely that the surface of the piezoelectric substrate will also be partially etched during the etching process, although the IDT is only required to be etched. As a result, it is impossible to adjust the frequency characteristic of an elastic surface acoustic wave device such that the adjustment is carried out as freely as desired.
Additionally, a method that is capable of providing a high treatment precision is the Reactive Ion Etching (RIE) method. However, the RIE method requires that the whole wafer be etched in the chamber of the RIE device. So, if electrode films within a wafer do not have uniform thickness, it is impossible to effect a partial etching treatment. Thus, a uniform thickness is required to ensure as many good products as possible.
Moreover, when a wafer is divided into several chip units of surface acoustic wave elements, these chip units are required to be adjusted individually in the RIE method. However, this kind of adjustment is not very efficient so that it results in high production costs.
Further, the selective etching of the IDT or the piezoelectric substrate depends on the chemical properties of the IDT and piezoelectric substrate. As a result, there may not exist a suitable etchant or etching gas that can selectively etch either the IDT or the piezoelectric substrate.
In addition, with the RIE method, when an IDT made of Al is to be formed on a piezoelectric substrate made of quartz crystal, CF4 is used as the reactive gas to adjust the frequency. At this time, an etching treatment using CF4 on the quartz crystal and Al will result in the quartz crystal being etched more than the Al. As a result, the frequency will have to be adjusted by obtaining a condition that is substantially the same as in which the upper surface of the piezoelectric substrate shown in FIG. 6B is selectively etched. However, when such a method is used for adjusting the frequency, it is required that the piezoelectric substrate be etched in a large amount in order to obtain a large frequency change. On the other hand, if the piezoelectric substrate is etched too much and in too large an amount, the characteristic of the piezoelectric substrate will be deteriorated due to the etching treatment. As a result, an adjustment can only be performed in a range in which there is no characteristic deterioration. Consequently, a problem occurs in that the range for adjusting the frequency is very narrow.
To overcome the problems described above, preferred embodiments of the present invention provide a method of manufacturing a surface acoustic wave device in which the frequency of the surface acoustic wave device is adjustable over a wide range. The preferred embodiments also provide a method which allows the surface acoustic wave device to be adjusted for each discrete device, and to be adjusted while measuring the frequency, or such that the frequency measurement may be changed over to a frequency adjustment or vice versa.
One preferred embodiment of the present invention provides a method of manufacturing a surface acoustic wave element including the steps of providing a piezoelectric substrate having an interdigital transducer, where the interdigital transducer has a higher density than the piezoelectric substrate, and ion bombarding the interdigital transducer and the piezoelectric body simultaneously.
In another preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device includes the steps of disposing a metal film on a piezoelectric body, where the metal film has a higher density than the piezoelectric body, forming a plurality of interdigital transducers on the piezoelectric body, cutting the piezoelectric body into a plurality of surface acoustic wave elements such that each of the surface acoustic elements has at least one interdigital transducer, simultaneously etching the at least one interdigital transducer and the piezoelectric body, and packaging at least one of the surface acoustic wave elements.
In another preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device includes providing a wafer, forming a plurality of interdigital transducers on the wafer, forming a plurality of surface acoustic wave elements, each of the surface acoustic wave elements having at least one interdigital transducer, packaging at least one of the surface acoustic wave elements, and adjusting a frequency of the packaged surface acoustic wave element.
In preferred embodiments of the present invention, the surface of the piezoelectric body is also preferably etched since such a surface is similarly bombarded by the ions in the same way that the IDTs are bombarded. However, since the IDTs are made of a metal having a greater density than that of the piezoelectric body, the frequency change caused by the etching of the piezoelectric body is not as great as the frequency change caused by the etching of the IDTs. Thus, the net frequency change is preferably higher.
Further, it is possible to achieve a partial bombardment of the ions on the wafer so that only a partial adjustment is performed. For example, it is possible to select certain ones of the plurality of surface acoustic wave elements disposed on a wafer, thereby allowing adjustment of only the selected elements, or just select a desired IDT of a surface acoustic wave element and adjust the same.
As described in the above, with the method of the present invention, since the surface acoustic wave device is made by forming a metal film having a higher density than the piezoelectric body, and since ions are caused to physically bombard the elastic surface acoustic wave device so as to reduce the thickness of the IDT film, and since the IDTs have a significant effect on the frequency, it is possible to adjust the frequency without having to scrape off large amounts of the piezoelectric material as in the RIE method.
Further, since ions physically bombard the surface acoustic wave device so that the thickness of IDTs or a metal film is greatly reduced, it is possible to locally concentrate the high energy, thus making it possible to effect a frequency adjustment in individual elements or to effect a partial adjustment of the frequency in a shortened time period.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.